The present invention concerns devices for controlling a liquid crystal cell and, ore precisely, such devices for controlling liquid crystal display cells intended to display segments and/or symbols.
In the present description, such a cell will be designated xe2x80x9cLCDxe2x80x9d (Liquid Crystal Display).
Such LCD cells are commonly used in numerous applications, in particular pocket calculators, wristwatches and measuring instruments.
French Patent No. 2,712,109 discloses a conventional LCD cell. FIGS. 1 and 2 of the present description show respectively views of the front face of a first substrate 1 and the back face of a second substrate 2 of the LCD cell disclosed in the aforementioned Patent.
The substrate or plate 1 includes first electrodes (in particular those designated by the references 1.1 and 1.1a to 1.1c) which form the symbols such as xe2x80x9cONxe2x80x9d, xe2x80x9cOFFxe2x80x9d and xe2x80x9cAUTOxe2x80x9d, and second electrodes (in particular those designated by the references 1.2) which form segments grouped in the shape of an xe2x80x9c8xe2x80x9d to form the figure display. By way of example, electrode 1.1a forms the xe2x80x9cOFFxe2x80x9d symbol, and electrodes 1.1b and 1.1c form two segments in a same group of segments in the shape of an xe2x80x9c8xe2x80x9d.
The substrate or plate 2 includes third electrodes (in particular those designated by the references 2.1) arranged facing the first electrodes of plate 1, and fourth electrodes (in particular those designated by the references 2.2) able to be arranged facing the second electrodes of plate 1, as described hereinafter.
Plates 1 and 2 are separated from each other by a sealing frame 3 which forms, with the two plates 1 and 2, a cell within which a layer of liquid crystal 4 is enclosed.
Plate 1 also includes a plurality of contact pads (in particular those designated by the references 1.3 and 1.5) arranged on the exterior of frame 3, as well as conductive paths (in particular those designated by the reference 1.4) connected to the first and second electrodes. By way of example, contact pad 1.3 is connected to the three electrodes 1.1a to 1.1c by path 1.4. The contact pads can be connected to an electric voltage source, and said connection paths are arranged so as to provide voltage across said electrodes of plates 1 and 2, as is described in more detail hereinafter.
One will briefly recall how the layer of liquid crystals 4 of LCD cell 1 is activated or deactivated.
An electric voltage is applied across one of the first (or second) electrodes of plate 1 and the electrode of plate 2 which is arranged facing said electrode of plate 1. As a result, an electric field is generated between these two electrodes, and this field acts on the component formed by the liquid crystals present between such electrodes, so as to change the transmission or reflection characteristics of the light through this component.
One usually speaks of the activation of the LCD cell in the event that light is blocked, and the corresponding symbol and/or segment is displayed, and of the deactivation of said cell in the event that the light is reflected, and said symbol and/or segment is extinguished (positive contrast mode).
Generally, an LCD cell such as described in relation to FIGS. 1 and 2, is connected to a conventional control device or driving circuit arranged to provide electric control voltages (or control signals) able to cause activation or deactivation of the LCD cell.
Such a control device commonly supplies control signals which are multiplexed, with the object of reducing the number of connections between the control device and the LCD cell.
In order to achieve multiplexing of these control signals, and with reference again to FIGS. 1 and 2, each of the contact pads of plate 1 is connected to three of the first and second electrodes of this plate. By way of example, contact pad 1.3 which is connected to electrodes 1.1a to 1.1c, can receive in sequence three respective signals from the control device. Typically, electrode 1.1a receives a first signal (xe2x80x9c1xe2x80x9d or xe2x80x9c0xe2x80x9d) from the control device during the first phase of a multiplexing cycle, respectively to display or extinguish the xe2x80x9cOFFxe2x80x9d symbol formed by electrode 1.1a. Likewise, electrode 1.1b receives a second signal (xe2x80x9c1xe2x80x9d or xe2x80x9c0xe2x80x9d) during the second phase of the cycle, respectively to display or extinguish the segment formed by electrode 1.1b, and electrode 1.1c receives a third signal (xe2x80x9c1xe2x80x9d or xe2x80x9c0xe2x80x9d) during the third phase of this cycle, respectively to display or extinguish the segment formed by electrode 1.1c. 
A control device of this type is disclosed in Japanese Patent Application No. 9171376.
FIG. 3 of the present description shows the device 10 disclosed in the aforementioned Japanese Patent Application. This device is arranged to control the activation and deactivation of pixels of an LCD cell 12. Device 10 includes several control means 14 connected to substrate 16 by a plurality of connection lines 18. These connection lines provide to means 14 data originating from a memory 20, and representing the allocation of the control of the pixels to be activated.
Let us consider the case in which such a control device is connected to the LCD cell disclosed in relation to FIGS. 1 and 2, to control the activation or deactivation of the LCD cell. Let us suppose for example that one wishes to command the display of the xe2x80x9cOFFxe2x80x9d symbol, given that the xe2x80x9cONxe2x80x9d symbol is initially displayed.
Control device 10 then requires six multiplexing cycles in order to carry out the command to display the xe2x80x9cOFFxe2x80x9d symbol.
A first cycle is assigned to reading the states (xe2x80x9c0xe2x80x9d or xe2x80x9c1xe2x80x9d) of the control means of each of the three electrodes connected to contact pad 1.5, in order to know the state of display of the xe2x80x9cONxe2x80x9d symbol. The second cycle is assigned to deleting the states which were stored in said means, and read during the first cycle. The third cycle is assigned to writing the xe2x80x9c0xe2x80x9d state in the control means of the electrode forming the xe2x80x9cONxe2x80x9d symbol, to command the extinction of such symbol. The fourth cycle is assigned to reading the states of the control means of each of electrodes 1.1a to 1.1c. In the expected case in which the xe2x80x9cOFFxe2x80x9d symbol is not initially displayed, the fourth cycle is followed by fifth and sixth cycles. The fifth cycle is assigned to deleting the xe2x80x9c0xe2x80x9d state from the control means of electrode 1a and the sixth cycle is assigned to writing xe2x80x9c1xe2x80x9d state in said means, to command the display of the xe2x80x9cOFFxe2x80x9d symbol.
One drawback of a control device such as that described hereinbefore, lies in the fact that it does not allow the manufacturer to adapt the allocation of the command to an LCD cell, so as to control the display of the desired message simply and rationally. This is all the more of a handicap if the LCD cell is of the same type as that described in relation to FIGS. 1 and 2, i.e. an LCD cell including segments and/or symbols which one wishes to display periodically in applications such as wristwatches, measuring instruments and electronic games.
Moreover, in the event that such control devices are made monolithically in a semi-conductor substrate, new constraints and concerns, linked to the making of integrated structures, such as the complexity, space requirement and cost of such structures, then arise.
One object of the present invention is to provide a control device for an LCD cell, this device overcoming the difficulties and drawbacks of the aforementioned conventional devices, in particular a device which allows the control of a predetermined LCD cell to be rationalised.
Another object of the present invention is to provide such a device able to be made in a semi-conductor substrate, occupying a minimum surface area.
Another object of the present invention is to provide such a device capable of being made by conventional manufacturing processes in the semi-conductor industry.
Another object of the present invention is to provide such a device satisfying the conventional semi-conductor industry constraints and concerns as to complexity, space requirement and cost.
These objects, in addition to others, are achieved by the control device according to claim 1.
One advantage of the switching means of the control device according to the present invention, is that they are able to be switched in accordance with a circuit layout depending on the LCD cell to be controlled, so as to connect the paths which provide the same signals able to control different electrodes to cause the simultaneous display of segments and/or symbols. In other words, the control device has a configuration which depends on the LCD cell to be controlled, which tends to reduce the duplication of identical data, and which involves use of a smaller storage memory for the addresses and data to be provided to the control means of said device. There is a consequent reduction in the consumption of electrical power.
One advantage of the integrated structure including the control device according to the present invention, is that it is formed of components which can be made during conventional manufacturing processes.
One advantage of such a structure is that it can receive and supply signals compatible with a conventional central processing unit.